Devices, such as loop reflectors, ring resonators, or partial reflectors that reflect or return at least a portion of a transmitted signal back towards an originating generating source are well known in the art. In this regard see, for example, the book “Fundamentals of Optical Waveguides” by Katsunari Okamoto, Academic Press, 2000, at pages 160–165, describing ring resonators, “Fiber Loop Reflectors” by David B. Mortimore, Journal of Lightwave Technology, Vol. 6, No. 7, July 1988, pages 1217–1223, describing loop reflectors, and “Optical Fiber Filter Comprising a Single-Coupler Fiber Ring (or Loop) and a Double-Coupler Fiber Mirror” by Y. H. Ja, Journal of Lightwave Technology, Vol. 9, No. 8, August 1991, pages 964–974.
Referring now to FIG. 1, there is shown a schematic of an exemplary prior art loop reflector 10 comprising a 2×2 power splitter 11 and an optional delay line 12. The power splitter 11 has a first input/output port 11a that is coupled to receive a signal from a remote generating source (not shown) and return a reflected signal thereto via a path A, a second input/output port 11b, a third input/output port 11c, and a fourth input/output port 11d. The second and third input/output ports 11b and 11c are coupled to first and second input/output ports 12a and 12b, respectively, of the optional delay line 12 via respective paths B and C, and the fourth input/output port 11d thereof is coupled to provide an output signal from the loop reflector 10 via a path D to a downstream device (not shown).
In operation, a signal received at the first input/output port 11a of the power splitter 11 from the remote generating source via path A is split into first and second portions. The first portion is delivered to the second input/output port 11b and is transmitted via path B to the first input/output port 12a of the optional delay line 12. The second portion is delivered to the third input/output port 11c and is transmitted via path C to the second input/output port 12b of the optional delay line 12. Signals returned from the optional delay line 12 to the second and third input/output ports 11b and 11c of the power splitter 11 are each split into first and second portions, where the first portion is transmitted via path A back to the remote generating source, and the second portion is provided as the output from the loop reflector 10 via path D.
Referring now to FIG. 2, there is shown a schematic of an exemplary prior art two-port ring resonator 14 comprising a 2×2 power splitter 15. A first input/output port 15a of the power splitter 15 is coupled to receive a signal from a remote generating source (not shown) at a first input/output port 15a. A second input/output thereof 15b is coupled to provide an output signal from the ring resonator 14 via a path B to a downstream device (not shown). Third and fourth input/output ports 15c and 15d of the power splitter 15 are interconnected via a path C.
In operation, a signal received from the remote generating source at the first input/output port 15a of the power splitter 15 via path A is split into first and second portions with the first portion being delivered to the second input/output port 15b and transmitted via path B as the output signal from the ring resonator 14. The second portion is delivered to the third input/output port 15c and looped back to the fourth input/output port 15d via path C. When the second portion is received at the fourth input/output port 15d, it is split into first and second portions with the first portion being transmitted via the second input/output port 15b, and path B, as a component of the output signal from the ring resonator 14. The second portion is delivered to the third input/output port 15c and looped back to the fourth input/output port 15d via the path C to repeat the process. Each signal round trip in the loop, C, adds a component to the output signal. These components will add constructively or destructively at the output port, depending on signal wavelength. The resultant spectral response depends upon the coupling ratio and loop length.
Referring now to FIG. 3, there is shown a schematic of an exemplary four-port ring resonator 17 comprising first and second power splitters 18 and 19, respectively. Each of the first and second power splitters 18 and 19 have first, second, third and fourth ports 18a, 18b, 18c, and 18d, and 19a, 19b, 19c, and 19d, respectively, where the respective third and fourth input/output ports 18c and 18d, and 19c and 19d of the first and second power splitters 18 and 19, respectively, are coupled together. The first port 18a of the first power splitter 18 is coupled to receive a signal from a remote signal generating source via a path A. The signal received from path A is split into first and second portions where the first portion is directed to the second port 18b and provides an output signal from the ring resonator via a path B. The second portion is directed to the third port 18c and is transmitted via a path C to the third port 19c of the second power splitter 19. In the second power splitter 19, the signal received on path C is split into first and second portions where the first portion is directed to the first port 19a as a reflected signal from the ring resonator 17 via a path D. The second portion is directed to the fourth port 19d of the second power splitter 19 and is transmitted to the fourth port 18d of the first power splitter 18 via a path E where it is split; and first and second portions thereof are directed to the second and third input/output ports 18b and 18c, respectively. The second input/output port 19b of the second power splitter 19 would not normally have a signal directed thereto unless a signal was received at the second input/output port 18b of the first power splitter 18 from a remote device, or the first input/output port 19a of the second power splitter 19. Each signal round trip in the loop, optical path C→E, adds a component to the output signal at port 18b and to the reflect signal at port 19a. These components will add constructively or destructively at the output port 18b and reflection port 19a, depending on signal wavelength. The resultant spectral responses at the output port 18b and reflection port 19a depend upon the coupling ratios and loop length.
Partial reflectors have also been used in prior art stabilization systems as described in the copending application U.S. Ser. No. 10/776,808. In a prior art laser stabilization method, a laser source is coupled at its output to a reflection filter that selectively reflects back a part of the output of the laser sources toward the laser to stabilize the laser source's spectrum and power. The reflection filter sets both the wavelength and the amount of reflection used to feed back a signal to the laser source as found in, for example, Fiber Bragg Gratings (FBG) stabilized lasers. In such FBG system, the pump laser is connected to the FBG via a Polarization Maintaining (PM) optical fiber. The FBG provides the required reflection for stabilization of the FP laser chip. This method has been extensively used to stabilize a single laser source. Some multiple wavelength applications have also used this method to stabilize multiple laser sources using individual FBG for each laser source followed by a Wavelength Division Multiplexer (WDM) to combine stabilized laser source signals.
In an exemplary prior art stabilized laser system, an output/input facet of a laser is coupled to an input/output port of a transmission filter. The transmission filter is coupled at an output/input port thereof to an input/output port of a partial reflector. An output port of the reflector provides an output signal from the stabilized laser system. The transmission filter sets the wavelength, and the reflector sets the amount of signal reflection provided back through the transmission filter to the laser source. As was described in the copending application U.S. Ser. No. 10/776,808, when a portion of the signal filtered by the transmission filter is reflected by the reflector, it is again filtered by the transmission filter to provide a feedback signal to the output of the laser. It is found that, in response to the feedback signal, the laser source produces a wavelength shift in a first direction and generates an output signal that now peaks at a center wavelength that is shifted by an amount δw and is no longer at the desired wavelength output signal. As a result an excess loss is produced by the wavelength shift of the laser.
It is desirable to provide a reflection and transmission filter arrangement that can be used for various purposes as, for example, in a single or multiple laser stabilization system that reduces the excess loss for a single or multiple laser source stabilization system based on the use of a transmission filter of various technologies.